It seems to me that BPEL Designer is, in effect, a Domain Specific Language,
so could leverage the technology being brought together in the GMF project.
This would have a number of benefits including integration with other
EMF-based tooling.

This is a valid point and something we will look into. Given that we are
starting from a clean slate, we would like to take advantage of as many
eclipse frameworks as possible and we will have an evaluation process in
the early design stage to review GMF, EMF and other frameworks that would
help us accelerate the delivery and deliver a tightly integrated solution.